In a cellular network, such as one employing orthogonal frequency division multiplexing (OFDM) or orthogonal frequency division multiple access (OFDMA), each cell employs a base station that communicates with user equipment, such as a cell phone, a laptop, or a PDA, which is actively located within its cell.
Initially, the base station transmits reference signals (synonymous to pilot signals) to the user equipment wherein the reference signals are basically an agreement between the base station and the user equipment that at a certain frequency and time, they are going to receive a known signal. Since the user equipment knows the signal and its timing, it can generate a channel estimate based on the reference signal. Of course, there are unknown distortions such as interference and noise, which impact the quality of the channel estimate.
In an OFDM or OFDMA system, different user equipments are scheduled on different portions of the system bandwidth. The system bandwidth may be divided into frequency-domain resource blocks of a certain size (sometime referred as sub-band) wherein a resource block is the smallest allocation unit available in terms of frequency granularity that can be allocated to user equipment. Each resource block consists of NRB contiguous OFDM/OFDMA sub-carriers. While the size of different resource blocks can in general vary, it is preferred to impose the same size across resource blocks. Otherwise, the resource blocks size shall be as uniform as possible across the system bandwidth. A different user could potentially go on each of these resource blocks. In addition, a user can be scheduled on a portion of the system bandwidth having adjacent resource blocks inside. Non-adjacent allocation for each user is also possible.
The user equipment determines a channel quality indicator (CQI) for each of the resource blocks based on the channel estimation performed. The CQI employed can be a signal to interference noise ratio (SINR) after detection. The CQI can also be a certain type of quality measure such as mutual information. Other types of CQI that reflect the quality of transmission channel are also possible. Furthermore, the CQI for different resource blocks can also be jointly encoded and compressed. The user equipment feeds back the CQI for each resource block to the base station. A higher CQI for a resource block allows a higher data rate transfer of information from the base station to the user equipment.
For systems with multiple transmit and multiple receive antennas (also termed as multi-input multi-output (MIMO) systems), improved throughput and/or robustness can be obtained by employing transmit pre-coding. To apply a pre-coding on a MIMO system means that a certain transformation (typically linear) is applied to the data stream(s) prior to transmission via physical antennas. The number of independent data streams is termed the transmission rank. With pre-coding, the number of physical antennas does not have to be equal to the transmission rank. In this case, the pre-coder is a P×R matrix, where P is the number of physical transmit antennas and R is the transmission rank. Denoting the pre-coder matrix as W and the R independent data streams as an R-dimensional vector s, the transmitted signal via the P physical antennas can be written as: x=Ws.
Depending on the duplexing scheme, the pre-coder matrix W can be selected at the transmitter or receiver. For an FDD system where the uplink and downlink channels are not reciprocal, the pre-coder matrix W is more efficiently chosen at the receiver (user equipment) from a finite pre-determined set of matrices, termed the pre-coding codebook. Based on the channel estimate, the user equipment determines the pre-coder selection corresponding to the CQI in each resource block that is needed to allow an optimization of data throughput, for example. Therefore, the pre-coder is also a function of the channel and its quality. The same codebook-based pre-coding scheme can also be used for TDD or half-duplex TDD/FDD.
Once this is done, the user equipment will feed back to the base station for each of its resource blocks, the pre-coder and the CQI that will be achieved if that pre-coder is used for the resource block in the transmission of data. For example, in the context of the 3GPP E-UTRA system deploying a 5-MHz transmission, 10 user equipments having feedback information pertaining to 25 resource blocks requires that 250 units of information be fed back to the base station, just to schedule them. This requires a high level of operational overhead information.
In addition to the CQI and pre-coder selection feedback, the user equipment shall also select and feed back the transmission rank. While transmission rank selection may or may not be performed for each resource block, this constitutes to additional feedback overhead.
Accordingly, what is needed in the art is an enhanced way to reduce the amount of initial feedback required between user equipment and base station.